Multi-cylinder engines known as variable displacement engines (VDEs) may operate with a variable number of active or deactivated cylinders to obtain fuel savings. Therein, a portion of the engine's cylinders may be disabled during selected conditions such as low engine speed and engine load. For example, a VDE may operate on all cylinders when a heavy load is being demanded, such as during an acceleration event. However, during a light load or a low speed condition, when demanded engine torque is low, some of the engine's cylinders may be disabled, increasing engine efficiency and minimizing pumping losses. Some VDEs may disable a selected group of cylinders, for example, a bank of cylinders, via a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves, via selectively withholding spark to deactivatable cylinders, and/or via disabling the fuel injectors of the disabled cylinders, thereby improving the fuel economy of the vehicle. On the other hand, the engine may operate with all cylinders active (combusting air and fuel) during high driver demand/engine torque conditions. In this way, the engine may enter and exit cylinder deactivation modes to accommodate different driving conditions.
Over time and varying vehicle operating conditions, the mechanisms that actuate the deactivatable cylinder valves (e.g., VDE mechanisms) may degrade, leaving the intake and/or may be impacted as the inability to seal the cylinder during deactivation results in pumping losses. Therefore, it may be desirable to determine whether or not VDE mechanisms controlling engine cylinders are operating as desired.
Various approaches have been identified for diagnosing degradation in a VDE system, such as based on manifold pressure. One example approach for diagnosing a VDE mechanism operation is shown by Bidner et al. in U.S. Pat. No. 5,721,375. Therein, detection of degradation of valve deactivation mechanisms is based on a pressure difference between an expected and an actual cylinder air charge manifold pressure during VDE operation relative to the pressure difference during non-VDE operation. Additional approaches for diagnosing VDE mechanism degradation and/or leaky cylinder exhaust valves may include the use of in-cylinder pressure sensors.
However, the inventors herein have recognized potential issues with such systems. As one example, in an engine with a large number of cylinders (e.g., six, eight, etc.), changes in engine airflow and/or manifold pressure reflective of a single cylinder valve degradation may not be identifiable. For example, in the event of an intake valve degradation occurring in just one cylinder of the engine, manifold pressure may not be impacted sufficiently to differentiate the change in manifold pressure resulting from the degraded intake valve from other fluctuations in manifold pressure. Further, fluctuations in engine airflow and/or manifold pressure not related to VDE degradation may confound the detection of VDE mechanism degradation during some conditions. Further still, the use of in-cylinder pressure sensors may be costly.
Thus, in one example, the issues described above may be at least partly addressed by a method including, during steady-state vehicle cruising, operating an engine with one or more cylinders of the engine deactivated, commanding the one or more cylinders to reactivate, and indicating cylinder valve actuator degradation responsive to a fuel usage change following the command to reactivate. In this way, a reliable diagnosis of VDE functioning may be made and degradation of the VDE mechanism controlling one or more cylinders may be determined.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.